Fail-fixed servovalve

ABSTRACT

A fail-fixed servovalve includes a jet pipe for discharging a pressurized liquid wherein the jet pipe may be selectively signaled to deflect and unbalance the pressures at opposing ends of a spool which would otherwise remain centered within a sleeve by resilient means at opposing ends thereof. The spool includes a plurality of circumferentially relieved areas interspaced between a plurality of lands with interconnecting passages therebetween wherein a servopiston may be actuated to move by pressurized liquid selectively received from the sleeve and spool such that a sudden loss of either the deflecting signal or the pressurized liquid would operate to block the flow of liquid to the servopiston, locking it in place.

United States Patent Kast 1 Dec. 2, 1975 I FAIL-FIXED SERVOVALVE 75 JInventor: Howard Berdolt Kast, Fairfield, "9' Maslousky Ohio Attorney,Agent, or F1rm lames W. Johnson, Jr.;

Derek P. Lawrence [73] Assignee: General Electric Company, Lynn,

Mass" 57 ABSTRACT Filedi Jan- 29, 1974 A fail-fixed servovalve includesa jet pipe for discharg- [211 App]. NO: 437,667 ing a pressurized liquidwherein the jet pipe may be selectively signaled to deflect andunbalance the pressures at opposing ends of a spool which would other-37/ 137/625-68 wise remain centered within a sleeve by resilient l5Clmeans at pposing ends thereof The spool includes a Fleld of Search tplurality of circumferentially relieved areas inter- 91/461 spacedbetween a plurality of lands with interconnecting passagestherebetween'wherein a servopiston may References Clted be actuated tomove by pressurized liquid selectively UNITED STATES PATENTS receivedfrom the sleeve and spool such that a sudden 2.796.851 6/1957 Ziskall37/625.68 loss of either the deflecting signal or the Pressurized 3133330 9 3 Rcitmunm" 25 51 liquid would operate to block the flow ofliquid to the 3.282283 11/1966 Takeda 91/461 servopiston, locking it inplace. 3,472,278 [0/1969 Arfelt 137/6242 3,528,446 9/1970 Horn l37/625.6x 7 Clam, 5 Drawmg F'gures Ara/21;;

US. Patent Dec. 2, 1975 Sheet 1 of 2 3,922,955

Jar/4y KER/Z US. Patent Dec. 2, 1975 Sheet 2 of2 3,922,955

- aaa FAIL-FIXED SERVOVALVE BACKGROUND OF THE INVENTION This inventionrelates to a fail-fixed servovalve and, more particularly, to afail-fixed servovalve which remains fixed in place upon loss of eitheran input signal or hydraulic liquid, and which may function as either adigital or analog device depending upon the frequency of the inputsignal.

Servovalves of the electrohydraulic type have been used broadly as theinterface between an electrical control signal and different types ofactuating and metering devices. Servovalves may also be directlyapplicable to the fuel control of a gas turbine engine. For example, ina gas turbine engine fuel control system, there may be an electricalsignal generated by a control which compares a reference engine speedwith an actual operating speed. This electrical signal may then beconnected to the input of a servovalve which in turn controls aservopiston wherein the mechanical output of the servopiston isconnected to a fuel metering valve. Thus the fuel flow of the gasturbine engine can be varied as a function of the electrical signal inorder to maintain the reference engine speed. Such a system wouldprovide a highly stable and accurate control of the engine speed.

Todays applications for servovalves, particularly in gas turbineengines, demand that the servovalve be failfixed. By fail-fixed, it ismeant that the mechanical output of the servopiston, as may be providedby an actuator, will be locked in place immediately following a loss ofeither the electrical input signal or the hydraulic lines. Present dayservovalves may have a mechanical bias which permits the servopiston tomove in a preselected direction upon loss of electrical signal. Thus inthe event of such a failure, the servopiston will be driven at apredetermined velocity in a preselected direction to the end of thepiston stroke. In the case of a fuel metering valve, the preselecteddirection would effect either a complete shutoff or maximum flow offuel.

There may also be an electrical failure wherein the servovalve receivesa constant maximum current signal of either a positive or negativepolarity. Present day servovalves which fail under this condition willprovide for a maximum flow to the servopiston so as to drive the pistonat maximum velocity in a direction determined by the polarity of thefailure.

Either extreme is obviously unsatisfactory and the better result wouldbe for the fuel metering valve to hold its initial position immediatelyprior to the failure as would happen if the servovalve were fail-fixed.

Electrohydraulic stepping motors provide many of the features ofservovalves and can be made fail-fixed; however, they are alsorelatively inefficient and incur many of the difficulties associatedwith servomotors.

Therefore it is a primary object of this invention to provide afail-fixed servovalve, the output of which will be locked in placeimmediately following the failure of either an electrical signal orhydraulic line.

It is also an object of this invention to provide a failfixed servovalvewhich retains the desirable features of conventional servovalves so thatpast experience and manufacturing techniques may be made applicable.

It is a further object of this invention to provide a failfixedservovalve which may be used in the fuel control of a gas turbine engineto convert an electrical input signal to a mechanical output.

SUMMARY OF THE INVENTION These and other objects and advantages will bemore clearly understood from the following detailed description anddrawings, all of which are intended to be representative of, rather thanin any way limiting on, the scope of invention. The servovalve of thisinvention includes a deflecting means and a jet pipe for discharging ajet of pressurized liquid wherein the deflecting means is secured to thejet pipe so as to deflect the jet upon an input signal to the deflectingmeans. A sleeve is provided with a plurality of ports therethrough, oneof which receives an inlet flow of pressurized liquid. A spool istranslatably disposed within the sleeve wherein the spool is alsocentered within the sleeve by resilient means at opposing ends of thespool. The spool also includes a plurality 'of circumferentiallyrelieved areas interspaced between a plurality of circumferential landswith flow communication selectively established between certain relievedareas by passages internal to the spool. A pair of receiver passages aredisposed to accept an equal amount of liquid and equal recoveredpressure from the jet pipe when in the non-deflected position whereinthe receiver passages connect with opposite ends of the sleeve. There isalso provided a servopiston having a piston translatably disposed withina bore, both sides of which communicate with ports in the sleeve.

An input signal to the deflecting means will operate to deflect the jetpipe so that the receiver passages receive unequal amounts of liquid andunbalance the pressure at opposing ends of the spool to translate thespool during which time a pulse of pressurized liquid is ported to oneside of the piston which is brought into momentary flow communicationwith the pressurized liquid entering the sleeve by an internal spoolpassageway. At the same time a pulse of high pressure liquid is alsoported away from the opposing side of the piston by another internalspool passageway thus having the net effect of moving the piston adiscreet distance within the bore.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understoodupon reading the following description of the preferred embodiment inconjunction with the accompanying drawings.

FIG. 1 shows a cross-sectional view of the fail-fixed servovalve of thisinvention.

FIG. 2 shows a cross-sectional view of a portion of the fail-fixedservovalve of FIG. 1 in another mode of operation.

FIG. 3 shows a cross-sectional view of a portion of the fail-fixedservovalve of FIG. 1 in still another mode of operation.

FIG. 4 shows a cross-sectional view of a portion of the fail-fixedserovalve of FIG. 1 in still another mode of operation.

FIG. 5 shows a cross-sectional view of a portion of the fail-fixedservovalve of FIG. 1 in still another mode of operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a fail-fixed servovalve 10 comprising a flexible jet pipe 12together with a pivot seal 13 mounted in a housing 14. The jet pipe 12receives a pressurized liquid, which may be any suitable servo fluid,for discharge through a relatively small area nozzle 16 into a chamber18. The chamber 18 has an outlet 20 which connects by way of a returnconduit 22 to a reservoir of low pressure liquid (not shown). Thepressure drop across the nozzle 16 of the jet pipe 12 causes a dischargeof a high velocity jet of liquid into the chamber 18. A pair of receiverpassages 24, 26 are disposed to accept an equal amount of the highvelocity liquid jet when the jet pipe 12 is at its illustrated neutralposition. The receiver passages 24, 26 connect with opposite ends of asleeve 28 in which a spool 30 is slidably disposed. Means are providedto deflect the jet pipe 12 and are herein shown as a torque motor 32which is made responsive to electrical signals furnished through lines36. An armature 34 of the torque motor 32 is secured to the jet pipe 12and the pivot seal 13, and exerts a bending moment thereon whendifferential current is applied to the lines 36. The jet pipe 12 andpivot seal 13 exert a resisting moment which causes its displacement tobe directly proportional to the magnitude of the differential current.

The spool 30 is maintained at the median position within the sleeve 28by a pair of opposing springs 38, which are respectively engaged by oneend of the sleeve 28 and by a plug 29 threadably inserted at the otherend of the sleeve. The spool 30 includes a plurality ofcircumferentially relieved areas 41, 42, 43, 44, 45, 46 and 47 which areinterspaced between a plurality of circumferential lands 48, 49, 50, 51,52, 53, 54 and 55. Flow communication is provided between the relievedareas 43, 44 and by a passageway 58 which is internal to the spool 30.In like manner, flow communication is provided between the relievedareas 41 and 42 by the passageway 56 and between the relieved areas 46and 47 by the passageway 60, both passageways of which are formedinternal to the spool 30. An inlet conduit 62 furnishes a supply ofpressurized liquid from a source (not shown) wherein the pressurizedliquid enters the space defined between the relieved area 44 and thesleeve 28 by way of an inlet port 64. The pressurized liquid thereuponenters the space defined between the relieved areas 43, 45 and thesleeve 28 by way of the internal passageway 58. Pressurized liquid alsoexits from the space defined between the relieved area 44 and the sleeve28 by way of an outlet port 66 which communicates with a conduit 68 forsupplying the high pressure liquid to the jet pipe 12. There are alsoprovided two outlet ports 70, 72 in the sleeve 28 which communicaterespectively with the spaces defined between the relieved areas 41, 47and the sleeve 28, whereby low pressure liquid is returned to areservoir (also not shown). As is readily apparent, the return conduit22 is in flow communication with the outlet port by way of the spacedefined between the relieved area 41 and the sleeve 28.

There is also provided a servo piston shown generally at as including apiston 84 disposed for translation within a bore 86 from which extends aconnecting rod 88 in integral connection with the piston 84. The headside of the piston 84 receives an inlet flow of pressurized liquid froman inlet port 81 which communicates with a port 76 in the sleeve 28 byway of an interconnecting conduit 78. In like manner, the connecting rodside of piston 84 receives an inlet flow of pressurized liquid from aninlet port 79 which communicates with a port 74 in the sleeve 28 by wayof an interconnecting conduit 77. O-ring seals 90 may be provided toinsure that the piston 84 and connecting rod 88 both sealingly engagethe bore 86.

During operation, thejet pipe 12, when in the neutral position, as shownin FIG. 1, directs a high velocity liquid jet at both receiver passages24, 26 so that the pressures on opposite ends of the spool 30 are equal.The distance between the nozzle 16 and the receiver passages 24, 26 issuch that substantially all of the kinetic energy of the jet isconverted to pressure in the passages. When the passages 24, 26 arefull, the excess liquid flows through the outlet 20 to the conduit 22from whence the liquid is passed to a low pressure reservoir (notshown).

When a differential current is applied to the torque motor 32, the jetpipe 12 is deflected an amount directly proportional to the magnitude ofthe current differential. The high velocity jet from the pipe 12 thenimpinges to a greater extent on one of the receiver passages to increasethe pressure on one end of the spool 30 and urge it into motion.Assuming now that a step input of positive maximum rated differentialcurrent is applied to the torque motor 32, the jet pipe 12 will thendeflect in the direction of the receiver passage 26 which in turn willincrease the pressure on one side of the spool causing the spool 30 totranslate in the direction as shown in FIGS. 2 and 3. Before the spool30 engages the end of the sleeve 28, as shown in FIG. 3, there will be amomentary alignment, as shown in FIG. 2, wherein a pulse of highpressure liquid is ported to the connecting rod side of the piston 84from the inlet port 64 which is brought into momentary flowcommunication with the port 74 by the internal spool passageway 58. Inlike manner, a pulse of high pressure liquid is ported away from thehead side'of the piston 84 by way of the port 76 which is brought intomomentary flow communication with the outlet port 72 by the internalspool passageway 60. As is readily apparent, once the positive ratedcurrent is applied and the spool 30 begins to translate due to thedeflection of the jet pipe 12, it will not stop until engaging the endof the sleeve 28, as shown in FIG. 3, and thus the position as shown inFIG. 2 is illustrative of only the instantaneous alignment assumed asthe spool translates toward the end of the sleeve.

If the differential current applied to the torque motor 32 is returnedto zero current, then the jet pipe 12 will also return to the neutralposition so that the pressures on the opposing ends of the spool 30again become equalized. Thus, the spool 30 will be translated by thecoaction of the springs 38, 40 back to the median position as shown inFIG. 1, passing once again through the momentary alignment as shown inFIG. 2. A second pulse of high pressure liquid will be applied to theconnecting rod end of the piston 84 from the inlet port 64, which isbrought into momentary flow communication with the port 74 by theinternal spool passageway 58. In like manner, a second pulse of highpressure liquid will be ported away from the head side of the piston 84by way of the port 76, which is brought into momentary flowcommunication with the outlet port 72 by the internal spool passageway60. Thus it becomes immediately apparent that a step input of maximumrated differential current to the torque motor 32 will operate to causethe piston 84 to translate a discreet distance as determined by thevelocity of the spool, the area of the ports, and the liquid pressuredifferential. In other words, if the electrical input to the torquemotor 32 were a series of square waves of current, stepping from zero tothe maximum positive rated current and back to zero, then the piston 84would move in small incremental steps in one linear direction.

The direction of the servovalve piston 84 can be reversed by applying anegative maximum rated differential current to the torque motor 32. Anegative differential current operates to deflect the jet pipe 12 in thedirection of the receiver passage 24, thus increasing the pressure ofthe liquid on one side of the spool 30 so as to translate the spool inthe direction shown in FIGS. 4 and 5. FIG. 4 shows the instantaneousalignment between the spool 30 and sleeve 28 which allows a pulse ofhigh pressure liquid to enter the head side of piston 84 from the inletport 64 which is brought into momentary flow communication with the port76 by the interconnecting spool .passageway 58. In like manner, a pulseof high pressure liquid is ported from the connecting rod side of piston84 by way of the port 74 which is brought into momentary flowcommunication with the outlet port 70 by the internal spool passageway56.

If the differential current input to the torque motor is returned tozero, the jet pipe 12 will again return to the neutral position asshownin FIG. 1, thus equalizing the pressure on both sides of the spool30. The springs 38 and 40 will then operate .to return the spool 30 tothe median positions as shown in FIG. 1 whereupon a second pulse of highpressure liquid will be applied to the head side of piston 84, and acorresponding pulse of high pressure liquid will be ported from theconnecting rod side of piston 84 when the spool 30 momentarily alignswithin the sleeve 28 as shown in FIG. 4. Thus it is apparent that if thedifferential current is in steps from zero to the maximum negative rateddifferential current and then back to zero, the servopiston 80 will moveso as to extend the connecting rod 88 a discreet distance as determinedby the velocity of the spool, the area of the ports, and the liquidpressure difference. It is also apparent that if the input signal to thetorque motor is a series of square waves of current, stepping from zeroto the maximum positive rated current and back to zero, or stepping fromzero to the maximum negative rated current and then back to zero, theservopiston will move in either direction in a series of small discreetsteps.

As previously discussed, servovalves of this type have broad applicationin the fuel controls of gas turbine engines wherein they may act as theinterface between a digital electrical control and a metering valve ormechanical actuator. For example, in a gas turbine engine fuel controlsystem, the current input to the torque motor 32 may be an electricalsignal generated by a control which compares a reference engine speedwith an actual operating speed. The piston 84 may be connected throughthe rod 88 to a fuel meteringcontrol valve (not shown). Thus the fuelflow to the gas turbine engine can be varied as a function of theelectrical signal from the control to maintain the reference enginespeed. Such a control system would provide a highly stable and accuratecontrol of the engine speed.

The servovalve of this invention is also fail-fixed in that if thedifferential current applied to the torque motor 32 should, for somereason, fail to return to zero, the spool 30 will then remain at an endposition within the sleeve 28 and the piston 84 will remain locked inposition due to the alignment of the spool lands which block the flow ofliquid from the ports 74, 76. It will be further appreciated that thepiston 84 remains locked in position regardless of the polarity of thedifferential,

current at the time of failure. In addition, should there be a loss ofliquid pressure due to a rupture or break" in.

one of the conduits, the spool 30 will be returned to the mediumposition as shown in FIG. 1 by the co-action of the springs 38, 40. Thusa failure in either the electrical or hydraulic systems will not resultin unpredicted movements of the servopiston. I

If the input differential current to the torque motor 32 should beswitched too rapidly and exceed the frequency response time of theservovalve, then the servovalve will operate as an analog device in thefollowing manner. If the differential current input is varied rapidlyfrom zero to its maximum rated value, at a frequency exceeding thefrequency response time of the servovalve, then the spool 30 will assumethe position as shown in FIG. 2 and allow a maximum continuous flow ofliquid to the connecting rod side of piston 84, together with a maximumcontinuous flow of liquid away from the head side of piston 84. Piston84 will thus move at a maximum continuous velocity as determined by thesize of the ports and the pressure differen-' tial of the liquid. Shouldthe amplitude of the differential current be less than that of themaximum rated value, then the jet pipe 12 will not deflect to itsmaximum distance and the pressure differential operating on the spool 30will not be sufficient to achieve the full alignment of FIG. 2, as wouldbe required for maximum flow. Thus the flow of liquid to the servovalvepiston will be reduced, effecting a corresponding change in the velocityof the piston 84 which is an analog representation of the amplitude ofthe differential current applied during each cycle. Likewise, shouldthere be a reduction in the actual time in which the rated differentialcurrent is applied to the torque motor, as would happen if the rateddifferential current were applied for less than one-half of each cycle,then there will also be a corresponding decrease in the pressuredifferential across the spool 30 wherein the spool will fall short ofachieving the full alignment position as shown in FIG. 2. Thus the flowof liquid to theservopiston80 will again be reduced, causing acorresponding reduction in the velocity of the piston 84 which providesan analog output representative of the reduced time that thedifferential current is applied to the torque motor.

As will be readily appreciated, the differential current may also beswitched to a negative polarity at a rate exceeding the frequencyresponse time of the servovalve 10, in which case for a maximum ratedcurrent amplitude applied for the maximum time not to exceed one-half ofeach cycle, there is shown in FIG. 4 the full alignment position assumedby the spool 30. This position provides for the maximum continuous flowof liquid to the head side of piston 84 together with a maximum flow ofliquid away from the connecting rod side of the piston 84. Thus thevelocity at which the connecting rod 88 moves out of the servopiston 80'becomes an analog representation of either the amplitude of thedifferential current applied to the torque motor or the actual time thatthe differential current is applied. Again, it should be readilyappreciated that the servovalve can be operated only as an analog devicewhen the input differential current to the torque motor is switched at afrequency exceeding the frequency response of the servovalve. Otherwise,the servovalve will operate as the digital stepping device firstdescribed.

Thus the output flow of liquid from the servovalve to the servopiston isproportional to either the amplitude of the high frequency currentsignal or the actual time the current is applied during each cycle. Theservovalve therefore has a multiplication capability. For instance, onevariable such as servopressure may be used to vary the actual time thecurrent is applied during each cycle while a speed error signal could beused to vary the current amplitude during each cycle. The ability tosimultaneously vary two input signals to the servovalve permits the useof one signal to vary the gain of the servovalve for different operatingmodes of the system.

Accordingly, while the preferred embodiment of the present invention hasbeen depicted and described, it will be appreciated by those skilled inthe art that many modifications, substitutions, and changes may be madethereto without departing from the inventions fundamental theme. Forexample, conventional servovalves generally have a feedback springbetween the spool and jet pipe which could also be incorporated in thefailfixed servovalve of this invention.

What is claimed is:

l. A servovalve comprising:

a jet pipe for discharging a jet of pressurized liquid;

deflecting means responsive to an input signal for deflecting the jetpipe in a direction determined by the input signal; 7

a sleeve having a plurality of ports therethrough one of which receivesan inlet flow of pressurized liquid;

a spool translatably disposed within the sleeve and centered therein byresilient means at opposing ends thereof wherein the spool includes aplurality of circumferentially relieved areas interspaced between aplurality of circumferential lands and wherein selected relieved areasmay be placed in flow communication with selected ports in the sleeve bytranslation of the spool;

a pair of receiver passages in flow communication with opposite ends ofthe sleeve and disposed to accept an equal amount of liquid from the jetpipe when the jet pipe is in the non-deflected position and an unequalamount of liquid when the jet pipe is in the deflected position wherebyan input signal to the deflecting means operates to deflect the jet pipecausing the receiver passages to receive unequal amounts of liquid andthereby unbalance the pressure at opposing ends of the spool to causethe spool to translate in the direction of lower pressure; servopistonhaving a piston translatably disposed within a bore each side of whichcommunicates with separate ports in the sleeve; passage means internalto the spool and interconnecting selected relieved areas for deliveringa pulse of pressurized liquid to a first side of the piston and portingaway pressurized liquid from the opposite side of the piston when thespool is translated in one direction and delivering a pulse ofpressurized liquid to the opposite side of the piston and porting awaypressurized liquid from the first side of the piston when the spool istranslated in the opposite direction such that there is no fluid flow tothe servopiston when the spool is fully translated in either direction.

2. The servovalve of claim 1 wherein the spool includes: a firstcentrally relieved area for receiving the inlet flow of pressurizedliquid, second and third relieved areas each of which is adjacent onopposing end of the centrally relieved area and spaced apart therefromby first and second respective lands wherein the second and thirdrelieved areas are in flow communication with the centrally relievedarea by a first internal spool passageway, fourth and fifth relievedareas adjacent the second relieved area and spaced apart therefrom by athird land with a fourth land interspaced between the fourth and fifthrelieved areas wherein flow communication is established between thefourth and fifth relieved areas by a second internal spool passageway,and sixth and seventh relieved areas adjacent the third relieved areaand spaced apart therefrom by a fifth land with a sixth land interspacedbetween the sixth and seventh relieved areas wherein flow communicationis established between the sixth and seventh relieved areas by a thirdinternal spool passageway.

3. The servovalve of claim 2 wherein the sleeve includes:

a first port in flow communication with the space between the centrallyrelieved area and the sleeve for receiving the inlet flow of pressurizedliquid;

a second port spaced axially apart from the first port and in flowcommunication with one side of the servopiston wherein translation ofthe spool toward one end of the sleeve and back again operates toestablish momentary flow communication between the second port and thespace between the second relieved area and sleeve while translation ofthe spool in the opposing direction toward the other end of the sleeveand back again operates to establish momentary flow communicationbetween the second port and the space between the fourth relieved areaand sleeve; third port spaced axially apart from the first and secondports in flow communication with the other side of the servopistonwherein translation of the spool toward one end of the sleeve and backagain operates to establish momentary flow communication between thethird port and the space between the third relieved area and sleevewhile translation of the spool in the opposing direction toward theother end of the sleeve and back again operates to establish momentaryflow communication between the third port and the space between thesixth relieved area and sleeve; fourth outlet port spaced axially apartfrom the first, second, and third ports and in flow communication withthe space between the fifth relieved area and sleeve, and

a fifth outlet port spaced axially apart from the first,

second, third and fourth ports and in flow communication with the spacebetween the seventh relieved area and sleeve.

4. The servovalve of claim 3 wherein the resilient means are springsdisposed at opposing ends of the spool and the deflecting means is atorque motor having an armature secured to the jet pipe so as to exert abending moment thereon when differential current is applied to thetorque motor.

5. The servovalve of claim 1 wherein the input signal to the deflectingmeans is switched at a frequency exceeding the frequency response timeof the servovalve such that the spool assumes a position within thesleeve offset from the central position, but not abutting either end ofthe sleeve wherein a continuous flow of liquid is conducted to one sideof the servopiston while a continuous flow of liquid is ported away fromthe other side of the servopiston so as to impart a continuous movementto the servopiston, the velocity of the servopiston being an analogrepresentation of the magnitude of the input signal applied to thedeflecting means multiplied by the actual time the signal is applied.

6. The servovalve of claim 1 wherein the return to zero of the inputsignal to the deflecting means causes the jet pipe to return to thecentral position so that the receiver passages again receive equalamounts ofliquid and balance the pressure at opposing ends of the spoolsuch that the spool is returned by the resilient means to the centerposition within the sleeve during which time a second pulse ofpressurized liquid is ported to the same side of the piston previouslypulsed which side is once again brought into momentary flowcommunication with the pressurized liquid entering the sleeve by aninternal spool passageway while at the same time a pulse of highpressure liquid is again ported away from the opposing side of thepiston by another internal spool passageway thus moving the piston asecond discreet distance within the bore.

7. The servovalve of claim 2 wherein the servopiston remains fail-fixedin the event that the signal to the deflecting means remains at its fulldeflected position causing the spool to remain at an end position withinthe sleeve; said servopiston remaining locked in position due to thealignment of the spool lands which block the flow of liquid to and fromthe servopiston where the spool is in the end position.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 13,922,955 DATED December 2 1975 INVENTOR(5) I Kast, Howard Berdolt It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 8, line 6, change "on' to an Column 10, line 17, change 'vvtnere"to "When-- Signed and Scaled this second Day Of March 1976 [SEAL]Attest:

1. A servovalve comprising: a jet pipe for discharging a jet ofpressurized liquid; deflecting means responsive to an input signal fordeflecting the jet pipe in a direction determined by the input signal; asleeve having a plurality of ports therethrough one of which receives aninlet flow of pressurized liquid; a spool translatably disposed withinthe sleeve and centered therein by resilient means at opposing endsthereof wherein the spool includes a plurality of circumferentiallyrelieved areas interspaced between a plurality of cirCumferential landsand wherein selected relieved areas may be placed in flow communicationwith selected ports in the sleeve by translation of the spool; a pair ofreceiver passages in flow communication with opposite ends of the sleeveand disposed to accept an equal amount of liquid from the jet pipe whenthe jet pipe is in the nondeflected position and an unequal amount ofliquid when the jet pipe is in the deflected position whereby an inputsignal to the deflecting means operates to deflect the jet pipe causingthe receiver passages to receive unequal amounts of liquid and therebyunbalance the pressure at opposing ends of the spool to cause the spoolto translate in the direction of lower pressure; a servopiston having apiston translatably disposed within a bore each side of whichcommunicates with separate ports in the sleeve; passage means internalto the spool and interconnecting selected relieved areas for deliveringa pulse of pressurized liquid to a first side of the piston and portingaway pressurized liquid from the opposite side of the piston when thespool is translated in one direction and delivering a pulse ofpressurized liquid to the opposite side of the piston and porting awaypressurized liquid from the first side of the piston when the spool istranslated in the opposite direction such that there is no fluid flow tothe servopiston when the spool is fully translated in either direction.2. The servovalve of claim 1 wherein the spool includes: a firstcentrally relieved area for receiving the inlet flow of pressurizedliquid, second and third relieved areas each of which is adjacent onopposing end of the centrally relieved area and spaced apart therefromby first and second respective lands wherein the second and thirdrelieved areas are in flow communication with the centrally relievedarea by a first internal spool passageway, fourth and fifth relievedareas adjacent the second relieved area and spaced apart therefrom by athird land with a fourth land interspaced between the fourth and fifthrelieved areas wherein flow communication is established between thefourth and fifth relieved areas by a second internal spool passageway,and sixth and seventh relieved areas adjacent the third relieved areaand spaced apart therefrom by a fifth land with a sixth land interspacedbetween the sixth and seventh relieved areas wherein flow communicationis established between the sixth and seventh relieved areas by a thirdinternal spool passageway.
 3. The servovalve of claim 2 wherein thesleeve includes: a first port in flow communication with the spacebetween the centrally relieved area and the sleeve for receiving theinlet flow of pressurized liquid; a second port spaced axially apartfrom the first port and in flow communication with one side of theservopiston wherein translation of the spool toward one end of thesleeve and back again operates to establish momentary flow communicationbetween the second port and the space between the second relieved areaand sleeve while translation of the spool in the opposing directiontoward the other end of the sleeve and back again operates to establishmomentary flow communication between the second port and the spacebetween the fourth relieved area and sleeve; a third port spaced axiallyapart from the first and second ports in flow communication with theother side of the servopiston wherein translation of the spool towardone end of the sleeve and back again operates to establish momentaryflow communication between the third port and the space between thethird relieved area and sleeve while translation of the spool in theopposing direction toward the other end of the sleeve and back againoperates to establish momentary flow communication between the thirdport and the space between the sixth relieved area and sleeve; a fourthoutlet port spaced axially apart from the first, second, and third portsand in flow communication with the space between the fifth reLieved areaand sleeve, and a fifth outlet port spaced axially apart from the first,second, third and fourth ports and in flow communication with the spacebetween the seventh relieved area and sleeve.
 4. The servovalve of claim3 wherein the resilient means are springs disposed at opposing ends ofthe spool and the deflecting means is a torque motor having an armaturesecured to the jet pipe so as to exert a bending moment thereon whendifferential current is applied to the torque motor.
 5. The servovalveof claim 1 wherein the input signal to the deflecting means is switchedat a frequency exceeding the frequency response time of the servovalvesuch that the spool assumes a position within the sleeve offset from thecentral position, but not abutting either end of the sleeve wherein acontinuous flow of liquid is conducted to one side of the servopistonwhile a continuous flow of liquid is ported away from the other side ofthe servopiston so as to impart a continuous movement to theservopiston, the velocity of the servopiston being an analogrepresentation of the magnitude of the input signal applied to thedeflecting means multiplied by the actual time the signal is applied. 6.The servovalve of claim 1 wherein the return to zero of the input signalto the deflecting means causes the jet pipe to return to the centralposition so that the receiver passages again receive equal amounts ofliquid and balance the pressure at opposing ends of the spool such thatthe spool is returned by the resilient means to the center positionwithin the sleeve during which time a second pulse of pressurized liquidis ported to the same side of the piston previously pulsed which side isonce again brought into momentary flow communication with thepressurized liquid entering the sleeve by an internal spool passagewaywhile at the same time a pulse of high pressure liquid is again portedaway from the opposing side of the piston by another internal spoolpassageway thus moving the piston a second discreet distance within thebore.
 7. The servovalve of claim 2 wherein the servopiston remainsfail-fixed in the event that the signal to the deflecting means remainsat its full deflected position causing the spool to remain at an endposition within the sleeve; said servopiston remaining locked inposition due to the alignment of the spool lands which block the flow ofliquid to and from the servopiston where the spool is in the endposition.